VIII.] CLA Y SOILS 167 



special value both in supplying this needed constituent 

 and in bringing about the flocculation of the finest 

 particles, thus improving the texture and render- 

 ing the soil drier and warmer. Phosphoric acid 

 is also deficient in many clays, and as plants become 

 naturally somewhat shallow-rooted on such soils a 

 good supply of phosphate manure is often extremely 

 effective ; potash, on the contrary, is usually abundant on 

 clay soils. The typical crops of clay soils are wheat, 

 mangolds, beans, and permanent pasture, though the 

 latter may be very bad unless it is properly managed. 

 Weeds are not so abundant as on other soils, but a few 

 are specially troublesome, especially Black Bent Grass 

 {Alopecurus agrestis) and Field Mint (^Mentha arvensis) ; 

 the Rest Harrow {Ononis arvensis) is often a bad weed 

 on the poor pastures ; the Wild Carrot {Daucus carota), 

 the Teazel {Dipsacus sylvestris), and the Primrose 

 {Primula vulgaris) are characteristic. The Oak is the 

 typical tree of clay land, with Ash in wetter places, and 

 Hornbeam in the underwood and hedges. Clay land 

 pastures are often characterised by a very shallow- 

 rooting vegetation, which may even become largely 

 stoloniferous (creeping rooting), with such difficulty do 

 the roots penetrate the stiff, unaerated soil ; a form of 

 Bent Grass {Agrostis alba) often constitutes the bulk of 

 herbage; Sweet Vernal Grass {^Anthoxanthemufn odor- 

 atum) and Crested Dog's-tail {Cynosurus cristatus) are 

 also common. 



Calcareous soils may be either light, as when derived 

 from the Upper Chalk, or heavy and sticky, when they 

 contain much admixture of clay as they do on the Chalk 

 Marl and several of the argillaceous limestones in the 

 Midlands ; but in all cases they possess a very special 

 and characteristic flora. The wild plants are very full 

 of flowers, and the copses and hedgerows contain a 



